HashSet实现中的Null对象 [英] Null Object in HashSet implementation

问题描述

在Java API中，HashSet的实现是使用Object作为内部HashMap的值，

In the Java API, the implementation of HashSet is using an Object as a value for the inside HashMap,

   // Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();

public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

但HashMap允许其值为null。我认为没有必要填写这个值，为什么需要这个呢？

but HashMap allows its value is null. I think that's not necessary to fill the value, so why is this needed?

推荐答案

因为 HashSet contract指定 remove（）如果指定的对象存在并被删除，则返回 true 。为此，它使用包装的 HashMap #remove（）返回删除的值。

Because the HashSet contract specifies that remove() return true if the specified object existed and was removed. To do this, it uses the wrapped HashMap#remove() which returns the removed value.

如果你是要存储 null 而不是对象，那么对 HashMap #remove（）的调用将返回 null ，与尝试删除不存在的对象的结果无法区分，并且 HashSet.remove（）的合同可以不履行。

If you were to store null instead of an object, then the call to HashMap#remove() would return null, which would be indistinguishable from the result of attempting to remove a non-existent object, and the contract of HashSet.remove() could not be fulfilled.

这篇关于HashSet实现中的Null对象的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！